1. Field of the Invention
The invention relates to a method of measuring ambient temperature as well as to a temperature measurement arrangement for carrying out the method.
2. Discussion of the Background
Thermocouple elements, resistance thermometers, semiconductor elements, but also fiber optic sensors are in use as electric and electronic temperature sensors, that cover a large temperature range. Such types of sensors are generally formed as primary detectors arranged in the ambient medium, which can be supplied with voltage and current via electric lines.
For measuring the basis weight of high-speed conveyed material webs such as textile, paper and similar webs, a precise determination of the temperature of the ambient air is required for reasons of measurement accuracy. Here, the possibilities for using the mentioned off-machine temperature sensors are limited. The sensors must have a sufficiently short response time, and hence as low a heat capacity as possible, and on the other hand, must be sufficiently robust, since they will be mounted directly proximate to the material web, e.g. a web that is guided through a textile finishing plant in a harsh industrial environment.
As a rule, mechanical robustness in known embodiments is linked with a more solid construction and thus with too high a heat capacity and a correspondingly long response time.
The use of electrically exposed devices is moreover excluded or separate measures of protection are necessary in potentially explosive locations or in direct proximity to a material web that gives off inflammable solvent vapors.
Pyrometric temperature sensors that use a heat radiation spectrum and consequently operate in a non-contact manner cannot be used without problems for measuring the ambient temperature of a material web, since the heat radiation characteristics of gases, principally due to the very poor conformity thereof and which exhibit the behavior of a so-called full radiator, render a precise temperature determination very difficult.
The object of the invention is therefore to provide a method of measuring ambient temperatures, in particular directly proximate to high-speed material webs subjected to measurement of the basis weight, and a device for carrying out said method, which method or device may be implemented in a simple and cost-efficient manner in an already existing manufacturing process and/or control process allowing the temperature measurement to have a short response time, and which method or device guarantees a precise temperature measurement.
According to the invention, the temperature of a planar radiator arranged directly proximate to the material web and in thermal balance with the local environment, is measured across its heat radiation spectrum by means of a pyrometric radiation meter, and thus the temperature of the ambient medium directly above the material web is indirectly determined.
Since the radiator is in thermal balance with the ambient medium, the radiator exhibits the same temperature but with a selectively improved radiation characteristic. A pyrometric temperature measurement may be effected distant from the material web, whereby it is ensured by means of the planar radiator that the temperature of the ambient medium, e.g. air, is determined, and not that of an undesired background surface.
Concerning the arrangement, the radiator is formed as a thin sheet having a low heat capacity and, in relation to its volume, a large heat contact surface with the ambient medium. Thereby, a low response time of the entire temperature measurement arrangement is achieved, since the sheet adopts the ambient temperature within a short time.
To shield result-distorting heat radiation from other undesired heat sources, in particular from the direction of the material web, the sheet is provided, at least on one side, with a coating impermeable to at least the infrared spectral range of the electromagnetic spectrum. Heat radiation from heat sources other than the ambient air, is reflected by said coating, accordingly does not penetrate the sheet material, and thus heats the sheet only to a negligible degree.
For use as the shielding coating, metallic materials in particular are possible, which can be applied to the sheet, for example by a vapor or a similar deposition process. Metallic layers are therefore advantageous because these have a reflection coefficient of over 90% in the longer wave length range of the electromagnetic spectrum.
On the side facing the pyrometric radiation meter, the thin sheet may be provided with a an emission coefficient increasing coating of the sheet surface. The coating is effected such that the radiation characteristics of the sheet approximates as closely as possible to that of an ideal full radiator.
Previously, varnishes suitable for such a coating, in particular carbon varnishes, have been used, which have shown the highest possible conformity with the radiation characteristic of a full radiator in tests.
The entire device comprised of a thin sheet and a pyrometric radiation meter is formed so that the radiation meter is arranged spatially separated from the thin sheet. Here, the spacing is essentially determined only by the dimensions of the sheet surface and the aperture angle resulting from the pyrometer objective. This allows maintenance and repair operations to be particularly simple in that a defective sheet may be replaced with another sheet.
Measurement of the ambient temperature directly proximate to the material web from a distance is likewise possible, with all electric feed lines outside of the housing containing said pyrometer being no longer necessary.
The pyrometric radiation meter may be placed within a separately arranged housing that has, at least on one side facing the sheet, a window permeable to radiation in the infrared and longer wave spectral range.
The temperature measurement arrangement of the invention allows ambient temperatures to be determined without contact in the proximity of material webs being conveyed and/or moved. This is then advantageous in particular when the environment of the web is loaded with solvent vapors or other influences excluding standard temperature measurement with electric elements. Since the pyrometric measurement means is arranged outside of the environment to be measured, and the thin sheet does not require any electric feed lines, the risk of accident or interference is avoided.